This invention relates to a wooden roof truss and, in particular, to a wooden roof truss having a box frame construction in which readily available standard size lumber and plywood sheeting is utilized.
Johnson, in U.S. Pat. No. 3,346,999, discloses a wooden roof truss having a box frame construction. The truss is made up of three main sections including a vertical column section that is joined to a roof beam section by means of an elongated splice section. The splice section is placed at the same pitch angle as the roof beam and, in assembly, forms an extension of the roof beam. Each of the three sections contain spaced apart chords that form the end walls of the section and enclosing sheeting which forms the sidewalls of each section. Internal ribs are used to join the chords and thus provide additional strength to the structure. The column section of the truss contains a knee joint having a cylindrical compression block situated at the inside of the joint from which a series of studs radiate. Although the joint is relatively strong, most of the stress produced by loads are taken up by the elongated splice section, and in particular, by the joints between the adjacent column and roof beam sections. If the splice is not securely joined to the other two sections, high localized stress can build up in this critical region which can lead to failure of the truss.
It should be further noted that the three sections of the Johnson truss are co-joined at the time of erection by means of bolts. In this particular design, where the splice section is hung between the roof beam and column sections, the bolts are forced to carry at least a portion of the truss loading. The bolts therefore represent a weak link in the overall structure and thus limit the load carrying capacity of the frame. It should be further noted that the bolts can be easily bent or otherwise deformed if the frame twists or turns as it is being lifted into place at the time of erection.
Underhill, in U.S. Pat. No. 4,483,117, discloses a three part pre-fabricated truss that contains a top or peak section and two identical side sections which are nailed or bolted together in assembly. The sections are fabricated from wooden studs. The entire assembly is thus only as strong as the weakest stud section. Although the truss can be fabricated easily from standard size pieces of lumber, the overall span of the truss is limited as is its load carrying capacity.
Geffe, in U.S. Pat. No. 4,228,631, and Hunt et al., in U.S. Pat. No. 3,861,109, both describe composite wooden joists or beams which are suitable for supporting flooring or the like. Neither of these patents, however, are involved with roof trusses and it would not be possible to construct a roof truss using the teachings contained within these two patents.